Background: FIRE-4 (AIO KRK-0114) is performed in RAS wild-type (wt) mCRC patients. This randomized study tests the efficacy of early switch maintenance during 1^st-line therapy (part 1) and re-challenge with cetuximab (part 2) in later-line treatment. In part 1, all patients received first-line induction treatment with FOLFIRI plus cetuximab (FOLFIRI/Cet). In arm A, patients were randomized to continue FOLFIRI/Cet until progression or intolerable toxicity. In arm B, patients received FOLFIRI/Cet for 8-12 cycles, after which maintenance therapy with 5-FU/FA plus bevacizumab (5-FU/Bev) was applied. The first randomization evaluates the question if an early switch from cetuximab to bevacizumab during maintenance therapy may prolong PFS. The study protocol explicitly allowed a first cycle of chemotherapy to be applied before randomization. Methods: Within this randomized, controlled, open-label phase-III study, patients received FOLFIRI (irinotecan plus 5-FU/FA) plus cetuximab every two weeks at the standard dosing schedule. In arm A, FOLFIRI plus cetuximab was continued until progression or intolerable toxicity. De- and re-escalation was allowed according to the local standard of care. In arm B, patients received 8 cycles of FOLFIRI plus cetuximab (in case of tumor response) or 12 cycles (in case of stable disease) followed by maintenance with 5-FU/FA plus bevacizumab (5mg/kg) until disease progression or intolerable toxicity. Overall survival after second randomization (part 2) is evaluated as a primary endpoint. Here, we report PFS in first-line (part 1) as a secondary study endpoint of the study. Other secondary endpoints included ORR, OS, safety, and tolerability. Results: From August 2015 to January 2021, 672 patients were randomized, and 656 patients were assigned to treatment in 120 German and 10 Austrian centers (327 arm A and 329 in arm B). Of those, 205 patients received one cycle of FOLFIRI alone before randomization. In both arms, ORR was comparable for patients receiving cetuximab from the first cycle when compared to those receiving one cycle of chemotherapy only (arm A: 58.7% vs 62.9% (p = 0.54), arm B 60.2% vs 55.6% (p = 0.48). PFS was also not influenced in both arms (arm A: 10.8mo vs. 10.6mo (p = 0.91); arm B 11.2mo vs. 11.4mo (p = 0.62)). Preliminary results suggest that also OS (event rate 38.3%) was not influenced by one cycle applied without cetuximab (arm A: 33.7 mo vs. 29.1 mo (p = 0.20); arm B: 35.6 mo vs. 28.9 mo (p = 0.13)). Conclusions: Application of one initial cycle with chemotherapy alone did not influence the efficacy of a first-line strategy of FOLFIRI plus cetuximab. If RAS mutational analysis is not timely available, a start with FOLFIRI alone adding cetuximab in cycle 2 seems to be safe with respect of overall efficacy. Clinical trial information: NCT02934529.